This invention relates to a data processing apparatus and more particularly, it relates to a data recording/reproduction apparatus for recording and reproducing data in a high density and for effecting stable tracking. Furthermore, the present invention relates to an optical disk apparatus of a so-called "sample servo system" which effects recording/reproduction of signals by detecting clock marks, etc., disposed intermittently on tracks of an optical disk and to a recording/reproduction method for such an apparatus, and more particularly to a format structure and a recording/reproduction method suitable for increasing a storage capacity.
Various techniques have been proposed to accurately track data during high density recording and reproduction of data in an optical data recording/reproduction apparatus.
JP-A-2-183471 (corresponding to U.S. Ser. No. 07/460,370 filed on Jan. 3, 1990 now U.S. Pat. No. 5,233,589) proposes an optical data processing apparatus for recording and reproducing data in such a manner that a specific code word, such as "1", corresponds to front and rear edges of a recording pit, as an example of high density recording. To correct the change of edge positions of the pit caused at the time of recording data, or in other words, the change of a recording length from a normal length due to fluctuation of recording power and a recording pulse width, fluctuation of sensitivity of the recording medium, fluctuation of an intensity distribution of an optical spot for recording/reproduction, and so forth, this technique generates reproduction clocks from the front edge and the rear edge independently of each other, obtains a series of reproduction pulses and a series of reproduction clocks from the front edges and another series of reproduction pulses and another series of reproduction clocks from the rear edges, stores them in a memory in such a manner that the interval between the front and rear edges becomes a normal pulse gap, and effects data reproduction by an identification clock.
On the other hand, JP-A-60-167129 describes a data processing unit as an example of the technique for accurately tracking data. The optical disk medium described in this reference has track guide grooves which are wobbled (displaced) in a track scanning direction. When such an optical disk medium is employed, a track offset distance can always be detected even at the time of tracking or tracing the tracks, and can be corrected.
Another example of high density recording is known which uses a rotary recording medium on which a large number of tracks are disposed either concentrically or spirally at a constant pitch, and data are recorded on these tracks. To reproduce, erase or record data on an optical disk which is an example of the rotary recording medium, so-called "sample servo" has been proposed as a method for focusing control for keeping a focus of an optical spot on a recording surface of the optical disk, tracking control for tracking the optical spot on the track, or access control for selecting a track to which data are to be recorded. In the sample servo system, servo areas for detecting the servo signals at every predetermined angle of the optical disk are disposed and data areas for recording the data are disposed between the servo areas in the case of B-type format described in the 5-inch format JIS (JIS-X6271). A clock mark for generating a clock signal is disposed in each servo area and the off-focus signal, the offtrack signal or the optical spot moving signal for access is detected on the basis of the clock signal obtained from the clock mark to effect focus control, tracking control or access control. The same clock is also used for recording and reproducing the data.
The offtrack signal is generated from the difference between the reflected light powers when the optical spot passes through two wobble marks so disposed as to oppose each other while sandwiching the track center between them. Therefore there is an advantage that a detection optical system can be simplified. Since the servo area and the data area are separated from each other and signal detection is carried out on a time division basis, detection of the servo signal and recording/reproduction of the data can be carried out by the use of a single clock synchronized with the clock mark without mutual interference.
A set of the servo area and the data area is referred to as a "segment". Several segments together form a sector. The address of the track is recorded at the leading part of the sector, and this part is referred to as a "header". The servo area, the data area or the segment comprised of them and the leading part of the sector are linearly aligned in the radial direction. Similarly, the clock marks in the servo areas are aligned linearly in the radial direction. Since the optical disk rotates at a constant angular velocity, the clock marks appear with an equal time interval. Since the disk rotates at a constant angular velocity, this is referred to as a "CAV (Constant Angular Velocity) system".
In contrast to this CAV system, a system referred to as a "CLV (Constant Linear Velocity) system" is known. This system changes the number of revolutions of the disk depending on a track position so that the linear velocity becomes always constant. This system is used, for example, in a DAD (Digital Audio Disk). According to the CLV system, data can be written continuously at the same pitch from the inner periphery, for example, and a storage capacity can be increased. Since the linear velocity is constant in this system, a burden to the recording/reproduction system can be reduced, and this system can be used for a medium having a small recording power margin.
Reference may be further made to U.S. Ser. No. 07/551,808 filed on Jul. 12, 1990, now U.S. Pat. No. 5,193,034, which discloses a magnetic disk apparatus of a constant recording type, having a plurality of disks. At least one of the disk surfaces is a servo face divided into a plurality of zones. Servo data areas and clock data areas are alternately arranged on the servo face in its circumferential direction and the number of magnetic transitions representative of clocks in the clock data areas is different from each of the divided zones.